In a PON (Passive Optical Network) system, when an ONU (Optical Network Unit) as a subscriber side device is connected, an individual number of the ONU is registered in an OLT (Optical Line Terminal), which is a station side device. Conventionally, the OLT performs Discovery processing at an arbitrary timing to collect individual numbers (correspond to MAC addresses in E-PON (Ethernet (registered trademark) PON)) of ONUs and registers the ONUs in an OLT database (see Nonpatent Literature 1).
When collecting individual numbers of ONUs, the OLT does not allocate user data in a time window in which registration request signals from ONUs are expected to be received. This time window is called a discovery window. The discovery window needs to be opened for a time required to cover round-trip times for installed ONUs. However, when the discovery window is extended to cover round-trip times, available user bandwidths are decreased.
Patent Document 1 mentioned below describes a technique of setting a fixed delay for an ONU when the ONU is newly set up and detecting the ONU within a narrow discovery window, thereby saving the bandwidth consumed for the discovery window.
Meanwhile, the PON system is a system of providing subscribers with economical optical access by sharing an optical fiber among a large number of subscribers; the more subscribers share an optical fiber, the higher its economic effects will be. Accordingly, the PON system has been undergoing improvements on a further split-ratio increase and extension.
Patent Document 1: Japanese Patent Application Laid-open No. 2001-326666                Nonpatent Literature 1: IEEE802.3ah Clause 64, Multi-point MAC Control        